Imprint of galaxy clustering in the cosmic gamma-ray background

Abstract

Star-forming galaxies are predicted to contribute considerably to the cosmic gamma-ray background (CGB) as they are confirmed γ-ray emitters and are the most numerous population of γ-ray sources, although individually faint. Even though the Fermi Gamma-ray Space Telescope will be able to resolve few star-forming galaxies individually, their fractional contribution to the CGB should become far more significant than it was for past measurements of the CGB as many of the brighter, formerly unresolved sources will be resolved out. Thus, the clustering feature of galaxies imprinted on the CGB might be detectable by Fermi. In anticipation of such measurements, we calculate the predicted angular auto- and cross-power spectra of the CGB from normal galaxies. We find that the amplitude of the auto-power spectrum is smaller than that for other sources, such as blazars and dark matter annihilation; the shape is also characteristic. We also show that the cross-power spectrum with galaxy surveys features larger amplitude. Fermi should be able to detect the correlation signature in both the auto- and cross-power spectra at angular scales of ~1°–10° after 5 years of operation. Such a detection would be valuable in confirming the level of the star-forming galaxy contribution to the CGB and, more importantly, in serving as a tool in the effort to discriminate between possible origins of the CGB.